1. Field of the Invention
This invention relates to information storage media and, in particular, optical information storage media.
2. Art Background
A variety of media have been devised to store information such as digitally encoded information. Two primary requirements for such storage media are that the information is easily written and that it is essentially permanently recorded, i.e., it is possible to accurately read the stored information even substantially after it has been written. Exemplary of a medium which satisfies these requisites is magnetic tape. It is relatively easy to permanently write information on a magnetic tape. However, despite the widespread use of magnetic tapes, other media such as optical storage media have been proposed because they offer the potential for relatively higher storage densities, relatively higher data transfer rates and random access to stored information. In such media, an optical change is induced in the media to record the data. The data is read typically by using a laser beam and a light detector to observe this optical change. A variety of phenomena has been used to effect a readable optical change. For example, a change in absorption, reflection, or Faraday rotation has been employed.
Despite the potential advantages offered by optical media, some compromises are generally made in their use. It is desirable to have a medium which undergoes an optical change when subjected to a relatively low-power level of writing energy. However, generally if a material undergoes writing with low-power levels, it also has somewhat poorer storage stability. Thus, a compromise is usually made between the power required for writing and the storage stability achieved.
Attempts have been made to avoid this compromise while maintaining the desirable properties associated with optical media. For example, garnet magnetic recording materials which undergo an optical change through a change in Faraday rotation are described by J. P. Krumme et al, Journal of Applied Physics, 48(1) 366-368 (1977), by J. P. Krumme et al, Journal of Applied Physics, 46(6) 2733-2736 (1975), and in U.S. Pat. No. Re. 29,530 reissued Jan. 31, 1978. In the Krumme medium, CdS is placed in thermal contact with a garnet recording material such as Gd.sub.2.85 Bi.sub.0.15 Fe.sub.4.83 Ga.sub.0.17 O.sub.12 which is grown epitaxially on a single crystal substrate. Electrodes are positioned on either side of the CdS material. A voltage is applied across the electrodes and light is applied to the CdS material at points corresponding to the desired positions for recording information in the garnet material. The points in the CdS where the light is incident become relatively more conducting, pass a relatively high current induced by the voltage being applied to the electrodes, and thus is rapidly heated. The heat generated in the CdS also heats the thermally adjoining points in the garnet material reducing its anisotropy field. This reduction allows an applied magnetic field to reverse the magnetization in the area adjoining the illuminated region of the photoconductor. The reverse magnetization induces a corresponding change in Faraday rotation. A relatively large quantity of heat is produced in the photoconducting material through the use of a normal light power and through the applied voltage. As a result, writing with nominal power is possible, and in the absence of an applied voltage, the garnet material is stable. Thus, two of the properties desirable in a recording medium are available.
Nevertheless, the reported information storage density, i.e., 2.5.times.10.sup.5 bits/cm.sup.2, in the magnetic garnet writing medium is relatively poor when compared to other optical recording media which have high-storage densities, i.e., greater than 10.sup.7 bits/cm.sup.2 but which are designed to strike a compromise between stability and required writing power. See, for example, R. A. Bartolini, Journal of Vacuum Science and Technology, 18, 70 (1981).
Additionally, epitaxial garnet materials covering large areas have not been grown. Thus, although it is possible to avoid the compromise between nominal power writing and storage stability, it has not been possible to avoid this compromise and maintain relatively high-storage densities over a large area medium--an area greater than 100 cm.sup.2.